1. Field of the Invention
The present invention relates to a process and apparatus for producing a polycrystalline semiconductor, or more particularly to a process and an apparatus for producing a polycrystalline silicon semiconductor having a small strain under stress.
2. Description of the Related Art
Silicon is an excellent raw material for production of industrial products, and is used, for example, as a semiconductor material for preparation of ICs (integrated circuits), etc., and as a material for preparation of solar cells; it is a really excellent material from the standpoint of a resource that finds many applications in these areas. More specifically, silicon is the material used for almost all the solar cell materials now in practical use. The currently dominating solar cells for power supply are based on single-crystal silicon, and thus further development of solar cells made of high-quality polycrystalline silicon is neccessary for cost reduction. Under the circumstances, the conversion efficiency of polycrystalline silicon is lower than that of single-crystal silicon. Therefore, development of a high-quality polycrystalline silicon is the foremost requirement for successful applications to solar cells.
According to the conventional process for producing polycrystalline silicon semiconductors, solid silicon inserted to a silica crucible is melted in a heating furnace, and is then cast into a graphite crucible. Another recently known method is melting in a vacuum or in an inert gas to prevent mixing of oxygen or nitrogen gas, etc. into the silicon to thereby improve the quality and prevent dusting.
In a semi-continuous casting furnace method of Wacker-Chemitronie GmbH in Germany, for example, silicon is melted in a vacuum or in an inert gas in a silicon crucible, and is then poured into a mold made of graphite or the like by inclining the crucible (Japanese Examined Patent Publication JP-B2 57-21515(1982)). In HEM (Heat Exchange Method) of Crystal Systems, Inc. in the U.S., silicon is melted in a vacuum in a silica crucible, and is then solidified directly (Japanese Examined Patent Publication JP-B2 58-54115(1983)). Also, there is known an improvement of the Wacker' process, wherein an water-cooled steel plate is used for the silicon melting crucible (Japanese Unexamined Patent Publication JP-A 62-260710(1987)).
In any above-mentioned silicon processes, the heat emission during solidification process of the silicon semiconductor is controlled for keeping it constant. As a result, when compared to initial stages where the silicon is solidified to transit to a solid phase from a liquid phase, more heat is transmitted through the solid phase thus occupying a large part in last stages. Since solid silicon has a higher thermal resistance than liquid silicon, however, it is difficult to discharge the heat emitted during the solidification process, resulting in a lower growth rate. Unless the growth rate is constant, a strain or a defect is liable to occur and deteriorates the crystal quality. For example, EPD (etch pit density), an index for evaluating the quality of crystal products, is normally about 10.sup.5 /cm.sup.2 for polycrystal silicon, which is much higher than the figure of less than 10.sup.2 /cm.sup.2 for single crystal.
In view of this, an attempt has been made to improve EPD by annealing. In the process disclosed by JP-B2 58-54115, for example, the crucible temperature is regulated for annealing after solidification (See the same patent publication, column 2, lines 33-36). A semiconductor ingot generally has a relatively large shape of about 30 to 50 cm square. The annealing of the s emiconductor ingot after the ingot has solidified, therefore, causes a temperature difference between the central portion and the peripheral portion of the ingot during the annealing process. The result is that the annealing process aimed at releasing the strain stress rather tends to generate a strain. The effect of the annealing process, therefore, cannot be substantially expected, and the polycrystal thus produced is of course liable to have a higher EPD.
For further improving the above-mentioned attempt, the applicant of the invention has applied for a patent on a process and an apparatus for producing a polycrystalline semiconductor having excellent crystallographic properties and having a small strain under stress by executing the solidification and the annealing alternatively (Japanese Patent application No. 7-344136(1995)). In this process, however, the heat emission is actually constant over the entire period from initial to last stages of the growth of crystal, resulting in a lower growth rate in the last stages than in the initial stages of solidification. Consequently, it is considered that one half of the effectiveness of the annealing is lost.